The present invention relates generally to cardiac pacing systems, and more particularly to a system and method for detecting cardiac contractions induced in response to the application of pacing pulses to a patient's heart.
While a variety of well known systems and methods exist for electronically detecting the naturally occurring contractions of a patients heart, the direct detection of externally induced cariac contractions has presented a significant technical challenge for which no completely satisfactory answer has yet been developed. Direct detection of an evoked cardiac response is frequently frustrated by the presence of strong pacing pulses which greatly exceed the magnitude of the cardiac signal to be detected. Cardiac event sense amplifiers are often saturated by such pulses and are thus rendered insensitive over a period of time during which induced cardiac response events most often occur. This particular difficulty has been largely overcome by the development of unsaturable, fast-recovery sense amplifiers such as that described by the present inventor in his copending application Ser. No. 738,608 entitled "Unsaturable Sense Amplifier for Pacer System Analyzer", filed concurrently herewith.
The direct detection of evoked cardiac response events has been further complicated by the presence of large post-pulse lead recovery artifacts which result from depolarization of the interface between a pacing lead and the adjacent cardiac tissue. Such recovery artifacts, which typically exceed the level of evoked cardiac response signals by several orders of magnitude, frequently mask the much lower level, and shorter duration, response signals which indicate the occurrence of induced cardiac events. Additionally, the large slew voltages developed by such artifacts require considerable system dynamic range, which is difficult to provide in a system intended to detect typically low-level induced cardiac response voltages.
One prior technique for directly detecting cardiac response signals involved the application of a post-pacing pulse reverse current to a pacing lead in order to more rapidly depolarize the lead-cardiac interface. This technique found application as a palliative, but as an "exact" lead depolarization technique suffered from the fact that charge discharge time constants are non-linear functions of pacer drive level, lead type, lead geometry, and time-modulated lead impedance. In addition, owing to the anodic voltages required, this approach introduced the possibility of inducing lead deterioration through corrosion.
Another prior technique was based on the observation that lead recovery artifacts correspond generally to the exponential decay characteristics of a resistor-capacitor network. Accordingly, in this sytem, the anti-log of the post-pulse signal was computed in order to recover what approximated the induced cardiac response. The accuracy of this system was inherently limited by the degree to which the lead recover artifacts departed from simple exponentials.
By contrast, the present invention is directed to a system and method for directly detecting cardiac contractions induced in response to application of pacing pulses to a patient's heart. The system effectively detects the actual cardiac response signal in the presence of both the applied pacing pulses and the subsequent resulting lead recovery artifacts. This system can be used with any cardiac lead configuration such as unipolar, bipolar, tripolar, etc., with any of the currently used lead materials such as platinum, elgiloy, etc., and with any of the lead tip geometries such as screw-in, ball-tip, parabolic, etc.
The system and method are particularly well adapted for use in a pacer system analyzer, wherein the operation of a cardiac pacer is evaluated in association with a patient's heart prior to implantation. By using a pacer system analyzer, a physician is able to tailor the operating parameters of a pacer system as required for the specific needs of an individual patient before the pacer has been fully implanted and the implantation surgery completed. The system and method of the present invention, when incorporated into a pacing system facilitates the measurement of a patient's capture threshold, which represents the minimum pace output level required to reliably stimulate cardiac contractions.
In view of the foregoing, it is a general object of the present invention to provide a new and improved system and method for detecting the occurrence of cardiac events
It is a further object of the present invention to provide a system and method for directly detecting the occurrence of evoked cardiac contractions.
It is still another object of the present invention to provide an improved system for promptly, accurately, conveniently and reliably determining the capture threshold of a patient's heart.